Orbit maintenance requirements for a spacecraft dictate the periodic need to fire the space craft thrusters to perform various maneuvers. One example is a stationkeeping maneuver requiring the firing of the thrusters on the East-West sides of the spacecraft (East-West maneuvers) or on the North-South sides of the spacecraft (North-South maneuvers). Propellant motion induced by the thruster activity necessary to perform these maneuvers produces disturbance torques on the spacecraft which degrade the spacecraft attitude pointing performance. These disturbances persist until natural dissipation effects in the propellant reduce its relative motion to quiescence. The disturbance torques affect the spacecraft attitude and, therefore, the spacecraft pointing performance.
To maintain the proper spacecraft attitude, during and after a stationkeeping maneuver, the attitude control of the spacecraft is changed to a special gyro-based attitude control scheme in order to meet the attitude pointing requirements of the spacecraft in the presence of the disturbance torques produced by the fuel motion. The special attitude control scheme requires the expenditure of a significant amount of propellant and may require the relaxation of the pointing performance specifications during and after the stationkeeping maneuver until the propellant has reached quiescence.
Another method used to reduce the disturbance torques during a stationkeeping maneuver is to use stationkeeping thrusters having very low thrust levels in order to minimize induced disturbances. Such thrusters, however, have limited utility during other mission phases such as the orbit acquisition phase or nutation damping phases. Larger redundant thrusters are needed to provide the same levels of thruster margin for the acquisition and nutation damping phases. The use of redundant thrusters increases the complexity and cost of the spacecraft.